1. Field Of The Invention
This invention relates to the removal of cyanide ions from aqueous streams, and more particularly to the removal of cyanide ions from industrial process waste water streams.
Cyanides are used in a variety of industrial processes that are carried out in aqueous media, such as the recovery of gold from ore and electroplating. Many gold mining operations use cyanide solutions to leach gold from the ore. After removal of the gold most of the leaching solution is recycled. However, a portion of the leaching solution is discharged as waste water to avoid accumulation of other metals. In metal plating, discharges of cyanide result from the rinsing of finished products from cyanide-type plating operations. This step likewise results in the discharge of considerable amounts of cyanide into waste water streams. Because of its extremely toxic nature cyanide in waste water streams must be destroyed before disposal of the stream.
2. Prior Art
Various techniques have been employed to destroy residual cyanides in waste water streams or to convert them to less toxic compounds. One technique commonly used is to convert the cyanides to less toxic cyanide complexes such as ferrocyanides and ferricyanides by the reaction of iron salts with the free cyanides. The resulting cyanide complexes are generally precipitated and removed from the aqueous stream as sludge. Unfortunately, this method of cyanide treatment results in the formation of considerable quantities of sludge. Furthermore, if the sludge is exposed to the environment the ferrocyanides and ferricyanides may be decomposed to free cyanides by the action of sunlight.
Another procedure for eliminating cyanides from aqueous streams involves the conversion of the cyanides to relatively harmless cyanates, which eventually break down and are released to the atmosphere as carbon dioxide and ammonia. Hydrogen peroxide, carbon dioxide and dilution water have been used in some western United States gold mining operations to reduce the amount of cyanide in tailings. The cyanide is apparently converted to cyanate. D. Muir, in an article entitled "Recent Advances In Gold Metallurgy", published in Symp. Ser. Australias. Inst. Min. Metall., 1987, 51 (Res. Dev. Extr. Metall.), 1-10, discloses the oxidation of cyanide to cyanate and eventually to carbonate and ammonia by contacting the cyanide with oxygen and carbon catalyst in the presence of Cu.sup.++ ions. Another method for converting cyanide in waste water streams to cyanate is disclosed in U.S. Pat. No. 4,537,686. The procedure described in this patent comprises contacting the cyanide with sulfur dioxide and oxygen in the presence of a copper catalyst, such as CuSO.sub.4. This process is effective, however it is costly and involves the use of sulfur dioxide, which itself is toxic and difficult to handle. Another possible disadvantage of this process is the fouling of process equipment as a result of the formation of insoluble compounds, such as calcium sulfate, when alkaline earth metal ions are present in the slurry.
U.S. Pat. No. 5,015,396 discloses the removal of cyanide from aqueous streams by contacting the streams with an oxygen-containing gas stream in the presence of carbon dioxide and a water-soluble reducible metal catalyst, preferably a metal catalyst selected from ferric salts and cupric salts. The preferred system of this patent is effective for removing cyanides from aqueous streams, but as shown in the example of this patent, the treated stream still contains a significant quantity of cyanide ion.
The growing use of cyanide in industrial water-based processes has resulted in the development of large volume cyanide-containing industrial waste water effluent streams and the creation of many cyanide-containing industrial waste ponds. These effluents and ponds present a constant hazard because of the danger of pollution of fresh water supplies from spills. Consequently, there is an ongoing need for more effective methods for removing cyanides from aqueous streams. The method of the present invention fulfills that need.